The technical field of this invention is phototherapy and, in particular, methods and devices employing optical fibers or other flexible light waveguides to deliver radiation to targeted biological sites.
Fiber-optic phototherapy is an increasingly popular modality for the diagnosis and/or treatment of a wide variety of diseases. For example, in surgery, infrared laser radiation is often delivered to a surgical site by an optically transmissive fiber in order to coagulate blood vessels or cauterize tissue. Similar fiber-optic delivery systems have been proposed for endoscopic or catheter-based instruments to deliver therapeutic radiation to a body lumen or cavity. U.S. Pat. No. 4,878,492 to Sinofsky teaches the use of infrared light to fuse fissures in the endothelial lining of a blood vessel during balloon angioplasty. U.S. Pat. No. 5,053,033 to Clarke teaches the use of ultraviolet light to prevent the proliferation of smooth muscle cells at an angioplasty site.
Fiber-optic irradiation has also been used to activate remote chemical agents with a patient's body. It is well known that light can promote photochemical reactions which, in the absence of light, would proceed either very slowly or not at all. The use of light to activate chemical agents within a patient is often referred to as "photodynamic therapy." For example, U.S. Pat. No. 4,336,809 (Clark) and U.S. Reissue Pat. No. RE 34,544 (Spears) disclose that hematoporphyrin dyes and the like selectively accumulate in tumorous tissue and that cancerous tissue that has taken up the dye can be preferentially destroyed by radiation (typically high intensity red light) absorbed by the dye molecules during phototherapy.
It has also been desirable to promote photothermal treatment for a variety of diseases. This involves the delivery of optical energy to the desired site and the conversion of that optical energy into thermal energy. The intense heat thus generated can cause undesired tissue to undergo necrosis or to separate from a substrate layer. In addition, high energy, rapidly pulsed laser radiation has also been proposed for essentially non-thermal ablation of tissue.
Typically, light can be delivered to the site of the desired phototherapeutic reaction by inserting a fiber-optic cable into a patient and maneuvering it to the site of the desired reaction site. It is often convenient to do so by passing the cable through a body lumen or by passing it directly into a body cavity. The position of the fiber optic cable inside the patient can be monitored by viewing it endoscopically.
In many phototherapeutic applications, it is desirable to uniformly illuminate a large region of tissue inside the patient. The highly-directional distribution of light exiting the fiber-optic cable is ill-suited for such applications. Additionally, the highly directional beam exiting a fiber-optic cable creates a risk of tissue damage by forming hot spots in the illuminated tissue. What is necessary and desirable in the field of phototherapy is an apparatus for the delivery of light having a uniform illumination field over a wide angle to the site of the desired phototherapeutic reaction. A particularly desirable uniform illumination field is one which is uniform over a sphere enclosing the phototherapeutic light source.
Both U.S. Pat. No. 4,693,556 to McCaughan Jr. and U.S. Pat. No. 5,429,635 to Purcell teach the creation of a uniform illumination field substantially as specified above by mounting a globular cap filled with a scattering medium to the tip of a fiber-optic cable having a radius smaller than the radius of the globular cap. Light entering this globular cap is as likely to be scattered backward in the direction of the fiber-optic cable as it is to be scattered forward. Since the globular cap extends radially well beyond the fiber-optic cable on which it is mounted, light scattered in the backward direction can continue to travel in the backward direction unimpeded by the presence of the fiber-optic cable.
Although the conventional globular cap can deliver the required illumination field, its unwieldy shape greatly hampers its usefulness in phototherapy. Because the diameter of the globular cap is so much greater than that of the housing on which it is mounted, it protrudes well beyond the walls of the housing. As a result, the globular cap tends to snag on the incision as it enters or exits the patient. This can result in damage to the diffusive tip assembly.
In addition, because the size of the globular cap is so awkward to maneuver, the surgeon may find it impossible to pass it through constricted spaces within the patient. As a result, the surgeon may find it necessary to reach the phototherapy site by a more circuitous path. This further increases the risk that the globular cap will snag on tissue as the surgeon maneuvers it through the patient. Where the phototherapy site is itself inside a constricted space, the size of the globular cap may make it altogether impossible to reach that site.
It is apparent that in order to scatter light in the backward direction, the conventional globular cap must extend well beyond the wall of the housing on which it is mounted. However the foregoing constraint practically ensures that the conventional globular cap will be extremely awkward to maneuver inside the patient.
Accordingly, it is an object of the invention to provide a diffusive tip apparatus for the delivery of an isotropic and omnidirectional illumination field to a phototherapy site without the danger of snagging tissue within the patient. Such a diffusive tip assembly can scatter light in the backward direction without the need to provide globular cap extending well beyond the wall of the housing on which it is mounted.